A new 20mph zone is likely to be established in Bath to allow council chiefs to assess whether such restrictions actually reduce speeds in residential areas.

Opposition Liberal Democrats on Bath and North East Somerset Council have been campaigning for more than two years to get such zones introduced more widely in the city.

They have pointed to the success of schemes to bring in multiple low-speed zones in cities such as Portsmouth, where there is a Liberal Democrat local authority.

Now, after attending a conference on the subject in the south coast city, the council's Conservative transport leader has ordered a trial project next year.

Councillor Charles Gerrish (Con, Keynsham North), the cabinet member in charge of highways and road safety, has asked council officials to investigate how 20mph zones have been implemented by other authorities - including Tory-controlled Oxfordshire County Council.

They have been asked to pinpoint possible locations for a trial 20mph zone in B&NES, which would begin next year.

Mr Gerrish said the authority had already invested heavily in road safety schemes, including some 20mph zones around schools.

There has long also been a 20mph zone in the Camden area of Bath.

Mr Gerrish said: "Following the success of these schemes, and having looked at other councils which have implemented 20mph zones in residential areas, we are keen to see a trial take place in B&NES and have asked council officers to put together possible locations for the cabinet to consider. Funding would then be earmarked for the trial in Conservative budget proposals for the next financial year."

The Conservatives say that one of the issues to be considered in choosing a location for the trial will be the current speed of traffic in different residential areas.

They say studies of areas which have already implemented 20mph zones have shown that where average speeds were above 24mph before the introduction of the new limit, speeds can be reduced by as much as 7mph.

But in areas where congestion or other factors have already slowed traffic below 24mph, average speeds were cut by less than 1mph.

There are also concerns over whether such new zones could be realistically policed.

Mr Gerrish added: "It's important that the right location for a trial is chosen. Studies into other 20mph zones have shown that they are most effective when coupled with other traffic calming measures, so it is important that 20mph limits are not implemented at the expense of other road safety schemes."

Liberal Democrats are still keen for the Tories to move to the more general policy advocated by their Twenty is Plenty campaign rather than what they call a "piecemeal" approach.

Councillor Caroline Roberts (Lib Dem, Newbridge) said: "The cabinet member should come out and admit that Twenty is Plenty and get on with Liberal Democrat proposals to slash speeds in all our residential areas.

"Liberal Democrats have been calling for the introduction of blanket 20 mph limits in residential areas for over two years, and there is a lot of support from residents for the idea.

"Such 20 mph speed limits would make walking and cycling safer and should result in fewer accidents."

The new limits in Oxfordshire have been criticised by the Oxford group of the Institute of Advanced Motorists, and the Oxford Driving Instructors' Association, with criticism that the programme was like "trying to repair a Rolex with a mallet" and had encouraged pedestrians to be more cavalier about crossing roads.